This application claims the benefit of the Korean Application No. P2000-78516 filed on Dec. 19, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) and, more particularly, to a cell switching method and a cell switching system.
2. Background of the Related Art
Generally, an ATM system divides user information into uniform packet sizes, generating cells of fixed sizes (53 bytes) by adding destination information to a header of each packet, and transfers the generated cells to a destination.
A protocol for transferring packet data in such an ATM system includes a physical layer, an ATM layer, an ATM adaptation layer (AAL), and an upper layer. The AAL layer divides and reassembles the packet data transferred from the upper layer and includes AAL1 to AAL5 types (see ITU-T I series recommendation).
The AAL2 protocol pack reduces the time taken to pack short segments of user data into an ATM cell by multiplexing or demultiplexing the short packets transferred from various users on an ATM network. This promotes the efficient use of the ATM network bandwidth. Namely, a basic concept of the AAL2 protocol is to multiplex or demultiplex various user data, which have short lengths as well as variable sizes, into a single ATM cell.
An AAL2 switch supporting the AAL2 protocol, according to the related art, will be described below.
FIG. 1 illustrates a block diagram of an AAL2 switch according to the related art. The AAL2 switch includes AAL2 transceiver units 10 that demultiplex an input AAL2 packet or multiplex the input AAL2 packet and an ATM switch 20 that switches an ATM cell at an ATM level.
The operation of the aforementioned AAL2 switch is as follows. First, an AAL2 receiving block in the AAL2 transceiver unit 10 demultiplexes the inputted cells, multiplexed as AAL2 common part sublayer (CPS) packets, into an AAL5 cell of 53 bytes. This is done to support packet switching at the ATM level.
Thereafter, the AAL2 receiving block transfers the converted AAL5 cell to the ATM switch 20. The ATM switch 20 transfers the AAL5 cell to an AAL2 transmitting block. Then, the AAL2 transmitting block de-converts and multiplexes the AAL5 cell into AAL2 CPS packets, so as to transmit the packets to a desired destination.
FIG. 2 illustrates a block diagram for explaining an AAL2 protocol recommended by ITU-T, according to the related art. An AAL2 protocol is divided into a service specific convergence sublayer (SSCS) and a common part sublayer (CPS).
Packet data of an upper application, in the form of service data units (SDUs), are transferred to an AAL through a service access point (SAP) of an upper layer (step S20). An AAL2 protocol SSCS then generates an SSCS-protocol data unit (PDU) by adding a header and tail to the SDU (step S21).
Successively, the CPS generates a CPS packet by adding a CPS header to the SSCS_PDU (or CPS-SDU), transferred from the upper layer, and generates a CPS-PDU of 48 bytes by adding a start field to the CPS packet (CPS-PDU payload) (steps S22, S23, and S24). In this case, the CPS-packet becomes a payload of the CPS-PDU. Since the CPS-PDU comprises 48 bytes, CPS packets of a plurality of users are multiplexed into the payload of the CPS-PDU (step S24).
Subsequently, the CPS-PDU is transferred to an ATM layer. Then, the ATM layer generates an ATM cell having a total size of 53 bytes, by adding a 5 byte header of destination information to the CPS-PDU (step S25). Thus, the packet data received from the upper layer are divided/assembled into 48 byte packets, through the AAL2 protocol, to be used for the payload of the ATM cell.
However, to communicate the AAL2 CPS packets across the ATM network, the ATM switch converts the AAL2 cell into the AAL5 cell, switches (i.e., communicates) the ATM cell to the corresponding destination, and then de-converts the ATM cell into the AAL2 CPS packets. Thereby, the ATM switch increases the complexity of the communication process as well as its processing time.
Moreover, the overhead of such an AAL2 switch degrades the quality of service (QoS) of a cell, thereby reducing the system efficiency.